ClassPool pool = ClassPool.getDefault(); CtClass ctClass = pool.makeClass("com.example.MyClass"); CtField field = new CtField(CtClass.intType, "myField", ctClass); ctClass.addField(field); CtMethod method = CtMethod.make("public void myMethod() { System.out.println(\"Hello, Javassist!\"); }", ctClass); ctClass.addMethod(method); CtMethod method = ctClass.getDeclaredMethod("myMethod"); method.setBody("{ System.out.println(\"Modified method implementation\"); }"); ctClass.writeFile();